The present invention relates generally to systems and techniques for improving RF (radio frequency) operating characteristics and protecting axial lead devices and more particularly to a conductive RF ground shield for improving RF operating characteristics and protecting conductor leads of axial lead devices, such as surface-mountable axial lead electromechanical relays.
Axial lead devices, such as electromechanical relays, for example, generally may be assembled to a printed circuit board (PCB) by inserting leads of the device into pre-drilled holes in the PCB and soldering the leads to lands (pads) in circuit traces on the PCB. Typically, the leads are soldered to the surface of the PCB opposite to the side on which the device is mounted. After soldering, excess leads extending beyond the solder fillet joints may be trimmed off close to the top of the solder fillet. This method of through-hole mounting adds manufacturing costs and has several disadvantages with respect to operating RF characteristics.
First, the through-hole assembly configuration has exposed solder fillets and residual short lead stubs. The residual lead stubs are the result of the traditional practice of lead trimming operations after soldering the leads to the PCB circuit traces and may affect RF performance characteristics. For instance, the solder fillets and lead stubs can radiate RF signals from one RF signal path to another RF signal path""s solder fillets and lead stubs. Similarly, signals from one RF signal path pole""s solder fillets and lead stubs can radiate and couple with other RF signal path poles"" (for multipole relays) solder fillets and lead stubs. Second, the PCB""s exposed RF signal circuit traces can radiate RF signals to other RF signal circuit traces and affect the relay""s operating RF performance characteristics. Finally, in the through-hole assembly configuration, the leads extend through the thickness of the PCB material. Hence, radiated RF signals from RF signal leads also can couple with other RF signal leads through the PCB bulk material, thus affecting the relay""s operating RF characteristics.
Attempts have been made to enhance the RF performance of through-hole assemblies, for example, by attaching ground connections to the device header base, cover, or other parts of the device enclosure and connecting such connections to the PCB""s RF ground plane. However, in these configurations, RF signals still can radiate from the leads of one signal path and couple to the leads of another signal path, adversely affecting the performance characteristics of the device.
Axial lead devices also may to be mounted to a PCB using surface-mount manufacturing technology. When axial lead devices, such as electromechanical relays, are soldered to a PCB by surface mount technology, the device enclosure is generally spaced off of the PCB by the leads. The relay leads attached to the PCB commonly use a butt-lead configuration (i.e., a short lead intended for attachment perpendicular to a land structure) or some variant of formed leads, such as the L-bend configuration. In the butt-lead configuration, the bottom end and part of the vertical surfaces of the leads are attached directly to the circuit trace solder lands (pads) on the PCB via the solder fillets. In this case, the solder fillets are formed between the butt leads and solder pads to create the structural attachments and electrical connections. In the formed lead configuration, the lower ends of the formed leads in contact with the PCB circuit traces are soldered to the circuit traces accordingly.
In either surface mount attachment configuration, however, exposed leads may affect the device""s RF performance characteristics. In the frequency domain, for example, the effects on the RF characteristics due to exposed leads in surface mounted axial lead electromechanical relays include, but are not limited to, reduced isolation across contacts, reduced isolation between poles (for multipole relays), increased return loss, and somewhat increased insertion loss beyond certain frequencies. Similar effects may be evident in the time domain with respect to parameters such as pulse rise time, propagation delay time, etc.
In general, degradation in RF performance characteristics may be due to RF signals that radiate off of one signal path""s leads and solder fillets and couple to another signal path""s leads and solder fillets. Radiated RF signals for a PCB signal path circuit trace to another signal path circuit trace, as mentioned above, on the PCB also affects the relays"" RF performance characteristics. To a lesser extent, there is some minimal RF signal leakage from one path to another through the body of the PCB dielectric material.
Attempts have been made to improve the RF performance characteristics of such surface-mounted devices by adding grounding attachments (e.g., connection ground tabs, ground straps) to connect the device""s conductive enclosure to the PCB""s RF ground plane. For surface-mounted axial lead electromechanical relays, attempts at improving the operating RF characteristics have included the use of discrete grounding means between the relays"" enclosure and the PCB RF ground plane. For example, grounding attachments have been added by welding one or more discrete ground pins to the bottom of the enclosure and soldering the bottom end of the ground pins to the PCB RF ground plane. This approach only partially shunts the undesired RF signals to the PCB""s RF ground plane. As such, these designs have provided only marginal improvement of RF operating characteristics over a limited RF frequency bandwidth.
The number of grounding attachments increases the assembly manufacturing complexity, and the operating RF performance of such configurations still has limitations over a broad frequency band. With either the butt-lead or formed lead surface-mount configuration, radiated RF signals still can travel from one signal path to another even if grounding means are attached to the device. As such, only a fraction of the radiated signals are shunted to the PCB""s RF ground plane. Similarly, radiated signals from one RF signal circuit trace on the PCB to another RF signal circuit trace may occur, even though some of the stray signals are shunted to the ground plane by a multitude of discrete grounding means attached to the device.
In one general aspect, an apparatus for protecting an axial lead device includes a conductive shield having one or more openings configured to receive a plurality of conductor leads exiting from the axial lead device. The conductive shield may be structured and arranged to substantially surround a plurality of conductor leads received in the openings such that each conductor lead is isolated from at least one other conductor lead. The conductive shield may shunt radio frequency signals carried by one or more of the plurality of conductor leads to a ground plane and isolate traces associated with the conductor leads when the axial lead device is mounted to a printed circuit board. Relative to an axial lead device when used without the conductive shield, an assembly including the axial lead device and the conductive shield demonstrates higher isolation of radio frequency signals across contacts of the assembly, higher isolation of radio signals across poles of the assembly, lower return loss of radio frequency signals, lower insertion loss of radio frequency signals, lower propagation delay time, and/or faster pulse rise time. The conductive shield may be structured and arranged for use with an off-the-shelf axial lead electromechanical relay device.
Implementations may include one or more of the following features. For example, the conductive shield may include a single conductive plate and/or two or more conductive plates. The conductive shield may be constructed from one or more of a plated metal, an unplated metal, a semi-metallic material, a dielectric material with conductive plating, a combination of these and/or any other material that provides the improved operating characteristics identified above. The conductive shield may have a thickness substantially equal to a length from the axial lead device to a bottom surface of at least one conductor lead and/or may have a square perimeter, a circular perimeter, a rectangular perimeter, and/or a multi-sided curvilinear perimeter.
The openings may include slots and/or corner openings. In some cases, a slot may extend into a central portion of the conductive shield. The openings may be closed by an outer edge of the conductive shield and/or be configured to provide clearance with a circuit board trace associated with a received conductor lead. The openings also may define a plurality of extensions. Each extension may isolate a plurality of conductor leads and may extend from a central portion of the conductive shield to the periphery of the axial lead device, to within the periphery of the axial lead device, or outside the periphery of the axial lead device.
The conductive shield may have one or more walls that are relatively perpendicular to a top surface and/or a bottom surface of the conductive shield. Such walls may form the boundaries of the openings. In some cases, the walls may be curvilinear and/or tapered. The bottom surface of the conductive shield may be substantially coplanar with a bottom surface of at least one conductor lead. The top and/or bottom surface may have a smooth surface finish, an embossed surface finish, and/or an intermediate surface finish.
In another general aspect an assembly may include an axial lead device having a plurality of conductor leads exiting from the axial lead device and a conductive shield. The conductive shield may include one or more openings receiving the plurality of conductor leads. The conductive shield may substantially surround the plurality of conductor leads received on the openings such that each conductor lead is isolated from at least one other conductor lead. In such an assembly, the conductive shield may provide a common electrical path between the axial lead device enclosure and a RF ground plane. The openings may be located at a periphery of the assembly.
Implementations may include one or more of the following features. For example, the axial lead device may be an electromechanical relay, such as a double-pole/double-throw relay, a double-pole/single-throw relay, a single-pole/single-throw relay, and a singlepole/double-throw relay, variants of the aforementioned contact configurations, and/or other multi-pole relays. Such electromechanical relays may include components such as an internal coil suppression diode, a polarity reversal protection diode, a transistor coil driver circuit, and/or an internal attenuator pad.
The axial lead device also may be surface mounted to a printed circuit board. The axial lead device may include leads having a butt-lead configuration, an L-bend configuration, and/or some other configuration. The axial lead device may be operable in a de-energized state and/or an energized state. The axial lead device may be a latching axial lead device, such as a magnetic latching axial lead device. The axial lead device may have a square lead pattern, a circular lead pattern, a rectangular lead pattern, and/or some other lead pattern. In some cases, the axial lead device may include a conductor lead that exits from a central portion of the axial lead device.
The conductive shield may be attached to the axial lead device by laser welding, resistance welding, soldering, use of a conductive adhesive, and/or some other attachment method. The conductive shield also may be integral with the axial lead device. For example, the conductive shield may form part of an enclosure of the axial lead device, such as part of the enclosure base of the axial lead device.
The conductive shield also may be attached by conductive means to a printed circuit board radio frequency ground plane. The conductive shield may be attached to the printed circuit board by one or a combination of: attaching by conductive means all of a bottom surface of the conductive shield to the printed circuit board radio frequency ground plane, attaching by conductive means the bottom surface of the conductive shield to the printed circuit board radio frequency ground plane at discrete locations, attaching by conductive means extension side surfaces of the conductive shield to a printed circuit board radio frequency ground plane, and touching substantially all of the bottom surface of the conductive shield to a printed circuit board radio frequency ground plane.